TreeDrop单选时报错

![image](https://github.com/wsfe/vue-tree/assets/25497779/113ff32e-8a63-418e-9446-833f4c1092fe)
![image](https://github.com/wsfe/vue-tree/assets/25497779/2ab418bc-8a37-4e1c-9214-34759b9613cd)
原因是$refs.treeRef 就不是RefImpl对象了，不需要.value，直接.titleField就好了